Well, the vet's right about cortisol levels being used to test for Cushing's, but I have no idea where he got the idea it would diagnose acromegaly. Cushing's involves excess cortisol levels... acromegaly is elevated insulin like growth factor (IGF-1) and growth hormone (GH) and the two diseases are very different.
Sure, you can test for Cushing's (although I believe I've read that the results of the cortisole levels can still not pinpoint if that's the issue--- other symptoms are generally apparent). You might also add in the test for insulin autoantibodies (IAA) as it's a pretty cheap test and can look like acromegaly because of the doses needed to overcome it (IAA is self-limiting-- it's not a permanent condition).
If you haven't already, read the sticky about acro and other high dose conditions... and the links in it, too, which can help explain each condition more fully.
Hope this helps...